Lumber from many tree species lacks durability and frequently has inferior physical properties. These deficiencies are more common in lumber extracted from man-made plantation forests.
It is typical for lumber processors to alter lumber properties to improve durability and enhance physical properties, such as hardness, water repellence, and protection against attack by insects or fungi.
It is well known to those versed in the art that these deficiencies can be remedied to a greater or lesser extent by impregnating the lumber with preservatives, polymers, and the like. Impregnation processes have been used for many decades. Most involve impregnation with treating fluids.
Typically the lumber is treated with waterborne preservatives or with solvent fluids based on non-polar organic solvents, such as white spirits (LOSP processes). Both processes are similar in that variations of vacuum and pressure are used.
When treating organic substrates it is essential that at least some of the biocide is available to attack the micro-organism—at least some of the biocide must be soluble. There is a compromise between achieving a very low level of solubility for health, safety and environmental reasons and retaining a level of solubility necessary for bioperformance.
It is known to those versed in the art of wood preservation that fungi attack substrates by a number of means including, for example, the production of extracellular compounds such as hydroxyl radicals, peroxides, strong acids, and the like. Some fungi produce extracellular oxalic acid, which can precipitate copper compounds and the like frequently used as wood preservatives, even at quite low pH. Once precipitated by oxalate the copper is no longer biologically active.
Historically one of the most important waterborne preservatives was CCA (copper chrome arsenate). However, arsenic is highly poisonous and the form of chromium used is carcinogenic.
Over the past two decades chromium and arsenic have in many instances been replaced by quaternary ammonium compounds, but because chromium was no longer present another method was required to solubilise the copper. One method involves use of large amounts of ammonia or organic amine compounds. Ammonia is a toxic gas and an environmental pollutant and organic amines are expensive. The use of these compounds is also wasteful, since most of such compounds are simply eluted from the wood after treatment. The preservative would be more competitive if these issues could be mitigated.
Preservatives based on copper solubilised using ammonia or amines are known generically as ACQ or copper azole. The preservatives typically contain a co-biocide based on a quaternary ammonium compound or an azole such as tebuconazole or propiconazole. The pH is typically around 10.5 to 11. The quaternary ammonium compound traditionally carried a halogen counter ion such as chloride, but in recent years this has been replaced by carbonate to reduce corrosion. Carbonate based quaternary ammonium compounds are more expensive therefore any means of reducing the amount used will provide value.
Important issues arise from the use of ammonia or amines. The ammonia or amines are critical to maintaining copper in solution. However, subsequent to treatment, residual ammonia or amines cause leaching of the copper from the wood. Leaching reduces wood loading and can potentially lead to failure of the wood. This issue can be addressed by increasing the initial loading in the wood at additional cost. Leaching of copper into the environment is, however, also problematic.
Another issue arising from the use of copper compounds solubilised by ammonia or amines is the corrosion of steel componentry used during installation of the treated wood. The corrosion is due to the soluble copper reacting directly with the steel. This has necessitated use of stainless steel fittings and fixtures which adds to cost. Corrosion has been mitigated to some extent by using halogen free compositions, but still remains an issue.
Another issue arising from the use of ammonia and/or amines is that these compounds provide a nitrogen source and thus encourage growth of disfiguring mould on the lumber surface.
Later versions of ACQ utilise amines. In earlier versions ammonia was used, but the excess ammonia released from the composition or the treated wood was a hazard to the health and safety of workers and the environment. Despite this ammonia remains in use for refractory wood species such as those encountered in Australia and the west of the United States because it facilitates penetration of treating compositions into these species.
The ratio of ammonia and/or amine or mixtures thereof to copper in the treating solution is typically a stoichiometric ratio of 4:1 of amine or ammonia to copper but this may be increased further to maintain solution stability. This is specified in standards such as AWPA P5-95 ACQ Type D.
The compound formed in the preservative solution is a tetraamine complex, for example, Cu(NH3)4, when ammonia is used—such ligand might be called a hard ligand.
A number of researchers have reported that the optimum ratio after treatment of the wood for best biological performance is a stoichiometric ratio of 1:1 of copper to ammonia or amine subsequent to leaching. This indicates that 75 percent of the solubilising agent is lost during leaching. The excess amine or ammonia is lost at a cost, and copper is also leached. If such leaching could be mitigated it would provide considerable benefit in terms of cost and safety.
U.S. Pat. No. 4,929,454 reports compositions containing copper compounds solubilised with ammonia and which include quaternary ammonium compounds.
U.S. Pat. No. 5,916,356 reports compositions containing copper compounds and azoles.
During the past decade a number of copper based preservatives that are not only free of chromium and arsenic but also free of ammonia and amines have been developed. This is achieved by converting the otherwise insoluble copper based biocide into a form in which the particle size is between around 100 and 1000 nanometers. Particles of this size are sufficiently small that, when impregnated using standard techniques, they can fully penetrate into the substrate. This approach mitigates the issues of leaching and cost due to the ammonia or amines.
However, these nano biocides have their own issues.
US 2008/0199525 reports the use of micronized biocides as wood preservatives. The biocides are prepared by milling using the likes of zirconia balls. The nano biocides are designed to eliminate the inclusion of costly and potentially toxic components, such as ammonia or amines.
Similarly, WO 2007/002156 reports a wood preservative prepared by grinding biocides in a ball mill in the presence of suitable dispersants. The use of dispersants prevents the particle size increasing through a process known as Ostwald ripening. Frequently, high loadings of dispersants are required to prevent agglomeration and settling of particles in sub-micron compositions. The use of dispersants increases the overall cost of the composition, particularly when high loadings are required.
US 2008/0199525 reports a reduction in the leaching of copper from treated wood samples by a factor of around 90 percent. This has cost and environmental benefits.
US 2008/0199525 reports a broad range of co-biocides which we incorporate herein by reference.
Claims are made commercially that the micronized copper based preservative systems are significantly less corrosive than the aforementioned ACQ types, including those based on halogen free quaternary ammonium compounds. Accordingly, in many service situations galvanised steel fixtures can be used, whereas stainless steel fixtures must be used in contact with ACQ treated wood. The reason for this is that micronized products have very low solubility and thus do not interact as corrosively.
Milling or grinding processes, however, have associated problems. On a commercial scale, equipment is capital intensive and can cost overall several million dollars. Energy costs may be high, cooling costs may be high and replacement grinding media is expensive. In addition, costly dispersants and surfactants may also be required.
Due to the capital required, plants need to be large and focus manufacture in key locations. This allows higher throughput to amortise capital costs, but can lead to high transport costs for the finished products to geographically remote users.
In addition, grinding is a very slow process, which exacerbates manufacturing times.
It can be seen that developments tend to mitigate problems but, frequently, at an additional cost.
Industry is striving to reduce the impact of compositions on workers and the environment without increasing costs.
It would be valuable to industry if the leaching, corrosion and mould issues could be mitigated without increasing cost or if at least an alternative to those currently available could be provided.
Typically micronized copper compounds include the likes of cupric hydroxide or basic copper carbonate. These compounds are generally prepared by the reaction of an alkali hydroxide or alkali carbonate with cupric sulphate. Cupric sulphate is one of the primary feeds in copper chemistry. But the step of converting the cupric sulphate to cupric hydroxide or carbonate introduces another process and cost.
In some circumstances cupric oxide can be used and when prepared by a direct oxidation route can be more competitive than cupric hydroxide or basic copper carbonate.
It is interesting to note that glycols have been used for many years in the preparation of boron containing biocides. NZ 549510 provides an extensive description of spiroboronate chemistry relating to the manufacture of soluble boron species using glycols, which is incorporated herein by reference.
EP 0046380 reports “a hygroscopic liquid carrier for application to a porous substrate such as timber wherein the composition contains at least 20% equivalent of boric oxide”. The use of various boron compounds is reported. The preferred embodiment uses disodium octaborate. A range of glycols are also reported. The preferred embodiment uses ethylene glycol. There is no mention of the chemistry involved, but it is likely that at least some spiroboronate or glycolate is formed.
U.S. Pat. No. 4,620,990 reports “a method of impregnating a wooden structure by diffusion of boric acid wherein solid rods of disodium octaborate are supplemented by an hygroscopic liquid which can be a glycol”.
U.S. Pat. No. 6,508,869 reports “a composition comprising an amine oxide and a boron compound.” This combination is designed to enhance penetration of boron compounds into lumber.
Those versed in the art will be aware that boric acid compounds, including borates and octaborates, form spiroboronate complexes with vicinal diols, such as glycols, and that in an aqueous medium this substantially reduces pH. This is typified in analytical chemistry wherein mannitol is added to borate solutions to allow the borate to be titrated as a strong acid.
Those versed in the art will be aware of the composition and structure of spiroboronates. An example is shown below (Spiroboronate Example 1). Borates can also form partial spiroboronates. An example is also shown below (Spiroboronate Example 2).